The present invention relates to new water-insoluble polymeric compounds which are soluble in aqueous alkaline solutions and which are suitable for use as binders for radiation-sensitive mixtures, in particular for the preparation of photoresist stencils and printing plates.
The polymers according to the present invention are particularly suitable for use in positive working radiation- or light-sensitive mixtures based on 1,2-naphthoquinonediazides, and in combinations of photolytically activatable acid donors with acid cleavable compounds. They are also suitable, however, for use as binders for negative working radiation sensitive mixtures.
Positive working light-sensitive mixtures, i.e., mixtures whose copying layer becomes soluble in the exposed area,, containing the aforementioned constituents are known. In a vast majority of cases, these mixtures contain phenol-formaldehyde condensation products, in particular novolaks, as alkali-soluble binders. Also used for this purpose are copolymers of maleic anhydride and styrene, of vinyl actate and crotonic acid, of alkyl methacrylate, butadiene and methacrylic acid (German Offenlegungsschrift No. 3,107,526) and polymers of vinylphenol (German Offenlegungsschrift No. 3,309,222). These polymers are evidently not preferred, since practically all known commercial products contain novolaks. Nevertheless, novolaks used as binders present disadvantages for certain purposes.
Due to the polyfunctionality of the phenols used, the condensation of phenols or creoles with formaldehyde under acid conditions leads not only to linear but also to highly branched and cyclic products which exert a strong influence on copying properties. The correlation of these properties with synthesis parameters is frequently very difficult or even impossible. This problem area is exhaustively discussed by T.R. Pampalone in Solid State Technology, June, 1984, at page 115 et seq. According to Pampalone, the material properties of novolaks are limited in variability and adaptability to practical requirements.
For use as binders in dry resist coatings, novolaks are too brittle because of their low molecular weight, so that a novolak-containing layer frequently breaks during processing. Layer breakage can occur, for example, in the course of exposure, when the layer is in contact with a mask, or in the course of lamination, when the layer is applied as a dry resist to a base-material surface. Layer brittleness is particularly noticeable in the case of the higher layer thicknesses preferred for dry resist materials.
Combined with o-quinonediazides, novolaks are used in positive printing plates. To increase the print run, the printing plates are baked, after exposure and development, at temperatures of about 200.degree.-250.degree. C. for a few minutes. In the course of this bake, low molecular weight constituents of the novolak can escape from the layer and deposit on the bared aluminum surface, which upon printing leads to toning, i.e., to undesirable ink acceptance in nonimage areas.
This poor thermal stability of novolaks is also responsible for image distortions in the preparation of integrated circuits by the dry etch method. In this method, temperatures can exceed 150.degree. C., and hence are distinctly higher than the softening point of the novolak. As a consequence, the image structures flow and lead to a lower resolution.
Polyvinylphenol is proposed in German Patentschrift No. 2,322,230 as a novolak replacement in layers that contain o-quinonediazides. Although the plasma resistance and the adhesion to silicon surfaces are basically comparable to novolaks, unexposed photoresist layers based on this polymer are excessively eroded in the course of development. It is true that the degree of erosion decreases with increasing molecular weight, but in the best cases (molecular weight .perspectiveto. 25,000) it is still higher than in the case of novolak-based layers by a factor of approximately 100 (according to T.R. Pampalone, Solid State Technoloov, June, 1984 at page 119). A further disadvantage is the "peparatively" expensive synthesis and the low polymerization tendency of monomeric vinylphenol.
Japanese Kokai No. 51/36,129 describes the use as binders of polymers based on hydroquinone mono-methacrylate and dihydroxynaphthalene monomethacrylate in naphthoquinonediazide layers. The synthesis of these monomers requires an expensive purification step, since the reaction of a diol with an acid chloride gives rise to the diester in a certain percentage, and this diester leads in the subsequent polymerization, even in amounts of 0.5%, to a complete cross-linking and, hence, insolubility of the polymer.